242 MR JOHN AITKEN ON THE 



other, were used ; these jets were enclosed in a dark lantern, having an opening 

 towards the dust-box. To concentrate the light, two double convex lenses were 

 fitted into a short tube. This tube was loosely attached to the front of the dark 

 lantern, and could be directed to, and focused on, any part in the interior of the 

 dust-box. For observing the phenomena two magnifying glasses were employed 

 — one a simple double convex lens, which was used for getting a general view 

 of the phenomena ; the other a more powerful compound glass, strong enough 

 to enable me to see and follow the movements of the individual dust particles. 



For observations on the effects of slight differences of temperature, metal 

 or glass tubes in some form or other were generally used. Straight tubes closed 

 at one end were found most convenient; these tubes were introduced through 

 the back of the box, and the closed end projected inwards to within a short dis- 

 tance of the glass front, so as to admit of observation under the strong magni- 

 fying glass. The tubes were heated or cooled by means of water or steam 

 introduced into them through a small tube which passed clown their interior. 

 This small tube was connected by an india-rubber tube to a glass filler, into 

 which the water was poured, and from which it flowed down the small tube to 

 the front end of the experimental one, and returned to the outside of the box 

 by the space between the tubes. In this way the experimental tube could be 

 easily heated or cooled, and the space all round it left free for observation. 

 For higher temperatures, a fine platinum wire, heated by means of a small 

 bichromate of potash battery, was employed. 



Different kinds of dust were used in the experiments, such as dust made 

 in the usual manner with hydrochloric acid and ammonia, and by burning 

 sulphur in the presence of ammonia; this last was used when very dense 

 fogging was required ; smoke of paper and other substances were experimented 

 with; also dusts made by burning sodium or magnesium; and for experiments 

 with dust which would not change with heat, calcined magnesia and lime 

 were employed. Charcoal powder was also used in some experiments. The 

 powders of these last three substances were stirred up by means of a jet of 

 air. These dusts were also varied by the addition of water vapour. 



Suppose now that the gas is lit in the lantern, and the dust-box in its place 

 Let us introduce into the box through the opening in the back one of the glass 

 or metal tubes, closed at the front end, and introduce into this tube from the 

 back the smaller one, and connect this latter with the filler, so as to enable 

 ms to pour hot or cold water through the tube, to heat or cool it. If we 

 are going to use smoke, a piece of smouldering brown paper is introduced into 

 the box, by removing the glass front, which is kept easily removable for this and 

 other purposes; or, if we are going to use sal-ammoniac dust, the ammonia 

 and hydrochloric acid can be introduced on glass or wooden rods through 

 small openings in the box, or the acid and ammonia may be placed in small 

 open ve3sels inside the box. If the dense sulphate dust is required, the sulphur 



